In modern immunizations either attenuated (weakened) live or killed viral or bacterial products are injected. The body reacts by making antibodies to those viruses and bacteria which afford protection against various viral and bacterial diseases.
In contrast, a homeopathic medicine is so diluted that no particulate matter remains. Often no molecules remain. In classical homeopathy, medicines are never injected but are given by mouth. Whereas a vaccine causes a limited and very specific reaction from the immune system (i.e., the production of antibodies), a homeopathic medicine elicits a nonspecific and more general reaction which stimulates the entire immune system as well as other systems.
In allergy desensitization, the allergist uses a dilution of an allergen (allergic substance), but he does not succuss it. Although the allergen is diluted, it is not diluted to the infinitesimally small dose the homeopath uses. Each desensitization shot acts only against one allergen whereas the homeopathic medicine strengthens the entire immune system against all allergens.